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Summary:

Although we live in an AC-dominated world, DC seems poised for a comeback, particularly in data centers. Facebook adopted a DC architecture in its Prineville, Ore., data center. SAP spent $128,000 retrofitting a datacenter at its offices in Palo Alto, Calif., to rely on DC power.

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In 1893, Rudolf Diesel was awarded a patent for the diesel engine. Gandhi committed his first act of civil disobedience. Thomas Edison created the movie studio. And zany New Zealand became the first country to give women the right to vote. Nabisco invented Cream of Wheat.

It was also the year that direct current (DC) took a back seat to alternating current (AC) after Niagara Falls Power Company chose AC transmission for its power plant.

Although we live in an AC-dominated world, DC seems poised for a comeback, particularly in data centers. Facebook adopted a DC architecture in its Prineville, Ore., data center. SAP spent $128,000 retrofitting a data center at its offices in Palo Alto, Calif., to rely on DC power. In 2010 it cut SAP’s energy bills by $24,000 per year.

ABB, the Swiss-Swedish conglomerate, bought a controlling interest last year in Validus DC Systems, which specializes in DC data center equipment. ABB also opened a factory in North Carolina to produce HVDC (high voltage DC) equipment for delivering power from solar and offshore wind farms to the grid. The Tres Amigas “superstation” will rely heavily on HVDC.

General Electric, meanwhile, bought Lineage Power, which produces DC equipment, and it has talked about using DC to power mining shovels and other heavy-duty equipment.

Nextek Power Systems and the EMmerge Alliance are also promoting DC as a way to cut power in buildings.

Behind the DC drive

What’s driving it? Although AC became the standard for electronic transmission, DC didn’t disappear. It just hid. Servers, large numbers of electric motors, batteries, even ships and airplanes run on DC. Solar panels produce DC power. Wind turbines can produce AC or DC power, but the extreme variability of wind power means that electricity generated by turbines has to pass through battery banks before it gets to the grid. As a result, wind farms are effectively DC.

The landline telephone system runs on DC too, notes Brian Fortenberry, a program manager at the Electric Power Research Institute.

To solve the mismatch, a whole industry of AC-DC converters has been developed. National Semiconductor sells billions of dollars’ worth of chips to convert power. Inverters in the solar industry exist to convert DC from solar panels to AC that can run on the wires in your home.

In data centers, the AC-DC gymnastics top the charts. Typically, AC from the grid has to be stepped down in voltage so it can be routed safely in building equipment. Lower-voltage AC then gets converted to DC so it can go to an uninterruptable power supply (UPS). DC power from the UPS then gets converted to AC so it can go over the wires in the building. Then it gets converted back to DC. Usually five conversions, or steps, downward take place.

By converting grid AC at the door of a data center to medium-voltage DC or converting stepped-down AC to DC at the last possible moment, a data center can cut utility bills by 10 to 20 percent or more, according to Trent Waterhouse, the VP of marketing for power electronics at General Electric.

Validus and others have also eliminated some of the technological hurdles involved in transmitting via DC, namely the monster-sized copper cables.

The same dynamics work in buildings. In a retail establishment, DC power from solar panels could go directly to DC-powered LED lights with not-intermediate conversions that sap energy, according to Nextek. Perhaps not coincidentally, Redwood Systems, the lighting networking company, touts that its technology is actually an example of DC networking.

More savings comes in real estate. DC data centers require 25 percent to 40 percent less square footage than their AC counterparts, largely because computer equipment can connect directly to backup batteries.

In a hypothetical example, a 2.5-megawatt data center power module in the AC world might need 7,295 square feet, Ronald Ranaldi, the VP of sales at Validus, told me last year. An equivalent DC power module might occupy only 5,102 square feet, a savings of 2,193 square feet. What’s more, a single data center might consist of several 2.5-megawatt modules.

“Real estate is often greater than the energy savings,” says Ranaldi. “In large, green field data centers, you are literally eliminating buildings.”

DC won’t take over the world. And not everyone is sold. Google is not taking DC for its data centers in part because of the cost that would be involved in retrofitting their existing architecture. But it seems that an idea that was current when Grover Cleveland was in the White House and Japan was just adopting the Gregorian calendar could make a comeback.

Image courtesy of The Planet

  1. Plaudit Design Monday, January 16, 2012

    I had never heard of the use of DC power in data centers. Interesting intro to the topic. Thanks

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  2. Uh, Facebook is using 480Y/277VAC for their distribution in Prineville, not DC. UPS power is through a rack mounted 48VDC system. Servers are dual power supply 277VAC and 48VDC respectively.

    see their post: http://www.facebook.com/note.php?note_id=10150144039563920

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  3. The main driver wasn’t the Niagara Falls power plant, though it had influence (it was 25Hz by the way), but the 1892 decision of the Chicago World’s Fair of 1893 (Columbian Exposition) to use the Westinghouse/Tesla AC powered lighting. Edison was not using any “higher” voltage, so his cost was 3x the price of Westinghouse. The organizers of the fair were already over budget and… they made the correct choice.

    Facebook in Prineville is most certainly NOT DC, it’s AC 277V with an “in row” UPS/Battery system 277/480VAC in 48VDC out for dual power supply 48VDC and 277VAC servers. This is part of their “open compute” source design:

    http://www.facebook.com/press/releases.php?p=214173

    DC requires smaller cable not “monster sized”, I think they’re confusing amperes with DC or AC. Current US Electrical Code does not allow use of smaller “DC only” cables, so there’s really no difference in cable sizes.

    The efficiency gains cited are incorrect, it’s in the mid to high single digits per most current accounts (5-7%). This has been disputed since the Lawrence Berkeley National lab published this in ’06 based on 15 year old technology. Green Grid wrote a counter proposal for the 5-7% which is the current accepted value.

    All of this is most certainly NOT to say there aren’t benefits of DC, which can be summarized as follows:
    Reliability due to fewer points of failure
    Smaller foot print
    Lower operation and maintenance costs
    Easily integrated into renewable energy schemes
    Efficiency (5-7% x 100MW = real money)

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  4. Davist
    I agree with your issue

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